1. Field of Invention
This invention relates in general to grid-like structures of the type suitable for use as collimators for shielding radiation receiving and imaging devices from the effects of distorting radiation, and more particularly to structures of the above type suitable for use in the provision more than one image of a given radioactive object at the same time.
2. Summary of the Prior Art
The use of grid-like structures formed of lead or some other material opaque to radiation as collimators in conjunction with various forms of radiation receiving and imaging equipment has been well known in the art for many years. One example of such use is the Anger camera, which is a special type of radiation receiver used by the medical profession to locate and judge the extent of diseased tissue within a patient's body by the creation of photograph-like images of radioactive concentrations therein. Recent advances have indicated that not only will radioactive material administered to a patient tend to collect in diseased tissue and thus be imagable, but also that such material travelling in the blood stream and through the various organs of a patient's body is also imagable so as to provide, particularly if used with a computer facility, a continuous series of images which together reveal the operation of a given organ. In each case, because of the peculiar fact that the object to be imaged is its own source of radiation, unlike the case of conventional or even x-ray photography, collimators have been invaluable tools for the selection from all the rays emanating from the radioactive concentration in all directions those rays which when allowed to reach the radiation receiving and imaging device will produce an undistorted image of the radioactive concentration.
Several types of collimators are known including the standard type wherein the walls of the grid-like structure and the holes defined thereby are all parallel and normal to the imaging crystal (image same size as radioactive concentration), those wherein the grid-like arrangements of walls and holes defined thereby either converges, or diverges, relative to a point on a line normal to the surface of the collimator (image either minaturized or enlarged relative to the actual size of the radioactive concentration), and the slant type wherein the walls and holes defined thereby are all parallel but are at an acute angle to the crystal (image same size as radioactive concentration). In addition, many hole shapes have been developed for and used in such devices, and many methods of collimator construction have been developed including modularization, and the design of special tooling for quantity manufacturing. The fact has remained, however, that collimator technology has imposed undesirable limitations upon the diagnostic capabilities of presently available radiation receiving and imaging devices.
For example, normally the radiation receiving and imaging device is located above the table or bed upon which the patient lies on his back, the radiation sensitive member (crystal) of the radiation receiving and imaging device and the bed/table defining two substantially parallel planes, however, it has been demonstrated in nuclear cardiac diagnosis that a view of the heart taken at approximately 30.degree. to a line normal to the plane of the bed/table provides a particularly useful and desirable view of the chambers, valves, and major blood vessels of the heart muscle. Using a standard parallel hole collimator, whose holes are normal to the camera crystal, to take this view, commonly called the Left of Right Anterior Oblique, is difficult because the camera must be positioned behind the patient's shoulder, and the resulting image is less than optimum due to the distance and the amount of tissue and bone through which the rays must pass prior to reaching the camera. The use of a slant type collimator whose holes are at an angle with respect to a line normal to the camera crystal has been found to be capable of yielding an image which closely approximates the conventional anterior oblique without the need to position the camera behind the patient's shoulder.
It has also been noted, again particularly in nuclear cardiac diagnosis, that simultaneous views of an internal human organ taken at different angles are an invaluable diagnostic tool, especially in cases wherein the physician needs as much information as possible as fast as possible. This is true not only in evaluating damage, say from a myocardial infarction, but also in examining present function, say by gated blood pool studies. As a practical matter many physicians in the field have endeavored to approximate the benefits of simultaneous imaging by serial multiple projections, that is taking a first view and then reorienting the patient, camera, or both before taking a second view. This practice, while useful, is not totally satisfactory because, among other reasons, there is a significant time delay between views and it is extremely difficult to achieve exact anatomical and geometric correlation between views so created. Only simultaneous imaging can effectively erradicate these problems.
Heretofore, simultaneous image studies have been possible only by the use of multiple radiation receiving and imaging devices using standard parallel hole type collimators or by the use of large field of view cameras fitted with a bi-lateral collimator. A bi-lateral collimator is one having two collimating sections, each section being essentially a separate slant type collimator. By making the angle of slant of each section of such a bi-lateral collimator with respect to a line normal to the camera crystal the same, for example, 30.degree. to the right of normal for the right hand section and 30.degree. to the left of normal for the left hand section, it is possible to create two simultaneous images of the radioactive concentration which convey relatively precisely correlated information in terms of time and space.
Such bi-lateral collimators are restricted in practical use. They allow multiple angular views of a radiation concentration to be created simultaneously on the same camera crystal, but they require large field of view cameras, (i.e., a large radiation sensitive member, commonly known as the crystal). For instance, in order to create two simultaneous views of the heart, which is about 15 cm in diameter, it is necessary using a bi-lateral collimator to provide a camera crystal approximately 15 inches in diameter. A camera of this size is simply too large and unwieldy to be considered portable and practically useable in an intensive care unit setting. The largest camera crystal which would allow its camera to be portable enough in an intensive care unit is about 10 inches. A large amount of imaging is done in intensive care units under rather severe time restrictions, especially in cardiac cases. Consequently, portability is a prime concern for any imaging system and as can be readily seen from the above collimator design can have a large effect upon imaging unit size.